What John Banks is labelling compelling new evidence for an appeal against his criminal conviction is believed to be sworn statements from two Americans whose presence at the critical donation lunch was a key issue at his trial.

But even as the shamed former politician pledges a legal challenge, his accuser, Graham McCready, has pledged to take another prosecution against Banks on the basis of new material obtained by the Weekend Herald.

The details, revealed through the Official Information Act, show the identity of the third mystery donor who contributed to Banks' Super City mayoral campaign - $500 million Rich Lister and Briscoes managing director Rod Duke.

Banks - former Auckland mayor, National minister and Act leader - was yesterday convicted of filing a false electoral return and sentenced to two months' community detention and 100 hours' community work.

Justice Edwin Wylie had found him guilty of filing the return without recording the origin of two $25,000 donations from entrepreneur Kim Dotcom, who is facing extradition to the United States.

His judgment hinged on the credibility of witnesses' evidence, telling the court he did not accept evidence from Banks or his wife, Amanda.

Outside the High Court at Auckland yesterday, Banks said: "Since the finding of guilt, fresh, new, unimpeachable, watertight evidence has emerged. That new evidence completely contradicts much of the evidence given in the court.

"We're looking forward to taking that ... to the Court of Appeal."

The Weekend Herald has learned details of the evidence that will be put before the Court of Appeal in a bid to have Banks' conviction overturned.

Evidence was given by the Dotcom camp, saying there were American guests on the grounds of the mansion on the day of the lunch but Mr Dotcom, estranged wife Mona and security chief Wayne Tempero said they were not at lunch when the donations were discussed.

The claims were contradicted by Banks in his police interview and Mrs Banks from the witness box. Both insisted there were two others at the lunch - businessmen Mrs Banks believed were Americans. Banks was unsure of their nationality.

Justice Wylie, who also took other evidence into account, said he believed the Dotcoms and did not accept the Banks' statements.

He also said it was "largely peripheral" because "the key issue was what was said at the lunch".

In the wake of the judgment, it is understood Banks launched a search for the American guests.

They were eventually tracked to a company in the United States which was a supplier for Mr Dotcom's defunct website Megaupload, which was brought down in the copyright raid that led to his arrest and possible extradition.

Banks' team was believed to have sought statements from them.

The new evidence claim was unusually raised by David Jones, QC, in his sentencing submissions to Justice Wylie yesterday. Mr Jones did not return calls but is understood to be filing the appeal papers next week.

Mr Dotcom said last night: "From my recollection, there was no one else at the lunch where the donations were discussed."

Meanwhile, Mr Duke - a Rich Lister with a retail empire - has been revealed as the third donor who police alleged should also have been declared. While police alleged the SkyCity donation should also have been declared, Justice Wylie found the charge was not proved. There has never been any suggestion that any of the donors acted improperly.

The name emerged after the Weekend Herald asked police to review deletions from the executive summary of their donations investigation. The main deletion was the name of a third donor who contributed $15,690 in radio advertising through the Radio Bureau.

It is the same document that sparked the private prosecutions against Banks. Mr McCready's prosecution targeted the SkyCity and Dotcom donations as they were revealed through the initial OIA requests. Mr Duke's name was withheld at the time but was released after the guilty verdict.

He could not be reached yesterday.

The document shows police interviewed Mr Duke about the donation.

He told them the cost of the advertising was discounted because of his Briscoes role and was recorded in the return as an anonymous donation and an advertising expense.

The true amount paid was actually $11,478.14.

The police analysis alleged "the donations for SkyCity, Dotcom and Rod Duke should not have been recorded as anonymous".

Mr McCready said yesterday that he would now take a prosecution against Banks for not declaring the Duke donation.